I am new to processing but have done a bit of programming in the past. I was making a 2d game and I need a picture to face where the mouse is pointing. So for example the picture is the top down view of a man's head and his head should rotate depending on where the mouse is located (i.e. his eyes face the mouse). 

I tried using tan to calculate the angle, but I got a weird result.

Here is my code (NOTE: only top right quarter of the screen can calculate the angle): 

2. PImage muha;
3. float rot;
4. float a;
5. float b;
6. 
   
7. void setup(){
8.   size(500, 500);
9.   muha = loadImage ("muha.png");
10.   
11. }
12. 
    
13. void draw(){
14.   background(200, 200, 200);
15.   
16.   if (mouseX > 250 && mouseY < 250){
17.     rot =  radians(90-atan((250 - mouseY)/(mouseX - 250)));
18.   
19.   }
20.   pushMatrix();
21.  translate(250,250);
22.   rotate(rot);
23.   image(muha, 0, 0);
24.   popMatrix(); 
25.  
26. }

 I would appreciate any help! 

Thanks,

Muhasaresa